Epidemiological evidence indicates a correlation between blood cholesterol level and sensorineural hearing loss. If a causal relation exists, it could be either that hypercholesterolemia affects auditory function and structure directly, or that the effect is indirect in that the ear merely becomes more susceptible to damage from occupational and societal noises, or both. This research will therefore determine both the direct and indirect effects of a high-cholesterol diet on auditory function and structure in an animal model. Groups of chinchillas will be subjected to a high-cholesterol diet for 3, 6 or 12 months; some groups will then be given noise exposures known to produce, in normal adult chinchillas, (1) severe temporary hearing loss but no permanent damage, or (2) moderate permanent damage. One day or 4 weeks after exposure, these indicators of auditory function will be assessed in the experimental groups, in nonexposed high-cholesterol controls, and in nonexposed normal-diet controls; thresholds and growth functions for cochlear microphonic, action potential, and brainstem response at 3, 4, 8 and 12 kHz; endocochlear potential; sodium and potassium concentration and oxygen tension in the cochlear fluids. Blood cholesterol levels will also be measured. At sacrifice, general atheromatous changes in the circulatory system and structural characteristics of the cochlea will be assessed using light microscopy. Appropriate comparisons will indicate the result of high cholesterol intake on the circulatory system in general and on the auditory mechanism in particular, and the effect of these changes on susceptibility to temporary and permanent losses from noise exposure. Subsequent experiments will determine the degree to which the effects are reversible; indicators of auditory function affected by the 12-month high-cholesterol diet will be evaluated after 3 months to 1 year of normal diet. Finally, the effectiveness, in ameliorating the changes induced by the high-cholesterol diet, of drugs that presumably enhance the oxygen supply to the cochlea will be determined.